1. Field of the Invention
The present invention relates to an FM receiver for receiving frequency-modulated (Frequency Modulation: FM) signals.
2. Description of the Related Art
An FM signal requires a wider frequency band than, e.g., an AM signal in order to be transmitted because the frequency of the carrier wave is varied based on the audio signal or the like. For this reason, an FM receiver is susceptible to interference (adjacent-channel interference) from other signals transmitted at a frequency adjacent to the frequency of the target signal when the target transmission signal is received. This exerts a negative effect on the quality of a detected audio signal. Adjacent-channel interference can be reduced by narrowing the band of a band pass filter (BPF) for extracting a receiving target signal (desired wave).
FIG. 3 is a block diagram for describing the configuration of a conventional FM receiver. The RF (Radio Frequency) signal received by an antenna is converted to an intermediate signal having a predetermined intermediate frequency (IF) fIF and inputted to an IFBPF2. The IFBPF2 is a band-pass filter whose center frequency is fIF, and the passband width WF is variable within a range of, e.g., about 40 kHz to about 220 kHz.
The FM signal that has passed through the IFBPF2 is fed to a limiter amplifier 4. The limiter amplifier 4 amplifies the FM signal to form a rectangular wave, and removes noise carried on the FM signal. The FM signal thus amplified and formed into a rectangular wave by the limiter amplifier 4 is inputted to an FM detection circuit 6. The FM detection circuit 6 detects a modulating signal in the output signal of the limiter amplifier 4.
An audio signal is reproduced based on the FM detection output. The FM detection output SDET is inputted to an adjacent-channel interference detection circuit 8 and a modulation degree detection circuit 10. The adjacent-channel interference detection circuit 8 extracts from the SDET the high band component that can be produced by adjacent-channel interference, and generates a DC signal SAI having a voltage level that corresponds to the strength of the high band component. The modulation degree detection circuit 10 generates a DC signal SMD having a voltage level that corresponds to the modulation degree of the received signal on the basis of the SDET.
The limiter amplifier 4 is also connected to a signal meter (S meter) circuit 12. The limiter amplifier 4 is composed of a circuit that connects several stages of buffer amplifiers in series. The S meter circuit 12 uses the output of each buffer amplifier of the limiter amplifier 4 as input signals and generates a received electric field intensity signal SFI that corresponds to the signal intensity of the FM signal on the basis of the input signals.
A bandwidth control circuit 14 receives as input the output SAI of the adjacent-channel interference detection circuit 8, the output SMD of the modulation degree detection circuit 10, and the output SFI of the S meter circuit 12, and controls the passband width WF of the IFBPF2. The bandwidth control circuit 14 works to narrow the passband width WF as the level of the output signal SAI of the adjacent-channel interference detection circuit 8 becomes higher. An FM signal from which adjacent-channel interference waves have been removed is inputted to the FM detection circuit 6, and degradation of the audio quality due to adjacent-channel interference can be reduced and controlled.
The bandwidth control circuit 14 works to narrow the passband width WF even when the received electric field intensity is in a predetermined weak electric field state. The weak electric field noise component that passes through the IFBPF2 can be reduced and sensitivity can be improved.
On the other hand, it is possible that audio distortion will occur when the passband width WF is narrowed in a high modulation state. In view of this possibility, the bandwidth control circuit 14 sets the passband width WF to a reference bandwidth even when the electric field is in a weak state in the case that adjacent-channel interference has not occurred and the modulation degree is high, and gives priority to voice distortion control over removal of weak electric field noise.
Control of the passband width WF for obtaining an advantageous audio quality is contradictory for the case of a weak electric field and the case of a high modulation degree. Accordingly, the WF is set to terms of a trade off between reduction of weak electric field noise and control of audio distortion, and an advantageous setting is difficult to obtain. The intensity of the adjacent-channel interference is also related to the setting of the WF, as described above, and, for example, it is not necessarily the case that, in a state of adjacent-channel interference, the adjacent-channel interference is removed in an advantageous manner by using the WF, even though the WF has been successfully set in an advantageous manner in the case of a weak electric field and high modulation without adjacent-channel interference. On the other hand, it is not easy to adjust the WF to an advantageous value with respect to the stated conditions, and the configuration of the bandwidth control circuit 14 becomes complex when an attempt is made to do so in a conventional bandwidth control circuit 14 using hardware. In other words, it is conventionally difficult to set the WF so that noise, audio distortion, and adjacent-channel interference are sufficiently controlled in various states of received electric field intensity, modulation degree, and intensity of the adjacent-channel interference. There is a problem in that the WF can only be set so that all of the characteristics are passably satisfied.